diff --git a/sandboxed_api/sandbox2/CMakeLists.txt b/sandboxed_api/sandbox2/CMakeLists.txt
index 27bc89b..984c23e 100644
--- a/sandboxed_api/sandbox2/CMakeLists.txt
+++ b/sandboxed_api/sandbox2/CMakeLists.txt
@@ -301,11 +301,11 @@ target_link_libraries(sandbox2_sandbox2
           absl::optional
           absl::str_format
           absl::strings
-          absl::synchronization
           sapi::strerror
           sapi::base
   PUBLIC  absl::status
           absl::statusor
+          absl::synchronization
           absl::time
           sapi::config
           sapi::file_base
diff --git a/sandboxed_api/sandbox2/sandbox2.cc b/sandboxed_api/sandbox2/sandbox2.cc
index ebbd9ab..d57e727 100644
--- a/sandboxed_api/sandbox2/sandbox2.cc
+++ b/sandboxed_api/sandbox2/sandbox2.cc
@@ -22,6 +22,7 @@
 
 #include "absl/memory/memory.h"
 #include "absl/status/statusor.h"
+#include "absl/synchronization/mutex.h"
 #include "absl/time/time.h"
 #include "sandboxed_api/sandbox2/monitor.h"
 #include "sandboxed_api/sandbox2/result.h"
@@ -44,14 +45,17 @@ absl::StatusOr<Result> Sandbox2::AwaitResultWithTimeout(
   if (!done) {
     return absl::DeadlineExceededError("Sandbox did not finish within timeout");
   }
-  monitor_thread_->join();
+  {
+    absl::MutexLock lock(&monitor_notify_mutex_);
+    monitor_thread_->join();
 
-  CHECK(IsTerminated()) << "Monitor did not terminate";
+    CHECK(IsTerminated()) << "Monitor did not terminate";
 
-  // Reset the Monitor Thread object to its initial state, as to mark that this
-  // object cannot be used anymore to control behavior of the sandboxee (e.g.
-  // via signals).
-  monitor_thread_.reset(nullptr);
+    // Reset the Monitor Thread object to its initial state, as to mark that
+    // this object cannot be used anymore to control behavior of the sandboxee
+    // (e.g. via signals).
+    monitor_thread_.reset();
+  }
 
   VLOG(1) << "Final execution status: " << monitor_->result_.ToString();
   CHECK(monitor_->result_.final_status() != Result::UNSET);
@@ -74,6 +78,7 @@ bool Sandbox2::RunAsync() {
 }
 
 void Sandbox2::NotifyMonitor() {
+  absl::ReaderMutexLock lock(&monitor_notify_mutex_);
   if (monitor_thread_ != nullptr) {
     pthread_kill(monitor_thread_->native_handle(), SIGCHLD);
   }
diff --git a/sandboxed_api/sandbox2/sandbox2.h b/sandboxed_api/sandbox2/sandbox2.h
index 6ec1db1..5f55530 100644
--- a/sandboxed_api/sandbox2/sandbox2.h
+++ b/sandboxed_api/sandbox2/sandbox2.h
@@ -27,6 +27,7 @@
 #include "absl/base/macros.h"
 #include "absl/memory/memory.h"
 #include "absl/status/statusor.h"
+#include "absl/synchronization/mutex.h"
 #include "sandboxed_api/sandbox2/comms.h"
 #include "sandboxed_api/sandbox2/executor.h"
 #include "sandboxed_api/sandbox2/ipc.h"
@@ -136,6 +137,9 @@ class Sandbox2 final {
 
   // Monitor thread object - owned by Sandbox2.
   std::unique_ptr<std::thread> monitor_thread_;
+
+  // Synchronizes monitor thread deletion and notifying the monitor.
+  absl::Mutex monitor_notify_mutex_;
 };
 
 }  // namespace sandbox2